Magnetic tape recording method and apparatus

ABSTRACT

A reel-to-reel direct tape drive mechanism is disclosed in which the data recorded on the tape is made insensitive to tape velocity variations during read and write operations by converting each coded character to be recorded into a unique bit count which is recorded so that only a counting operation is necessary to read data recorded on the tape.

Unite States tent [191 Thorpe [111 3,831,196 [451 Aug. 20, 1974 1 1MAGNETIC TAPE RECORDING METHOD AND APPARATUS Inventor: Allan ChesterThorpe, Raleigh, NC.

International Business Machines Corporation, Armonk, NY.

Filed: Aug. 25, 1972 Appl. N0.: 283,883

Assignee:

us. C1. 360/52 Hm. Cl. om s/oz Field QKQQhL1AQZ17.4 .Q 174: Hi AReferences Cited UNITED STATES PATENTS 11/1957 Burkhart 340/172.5 7/1969Schoeneman 340/1741 A 9/1969 Schoeneman 340/1741 A 3,474,429 10/1969MeCowen et a1 340/174.1 A 3,577,132 5/1971 Anderson 340/1741 A 3,601,8088/1971 Vlack 340/1725 3,614,757 10/1971 Burr 340/1741 A 3,631,42712/1971 Hein 340/1741 A 3,643,228 2/1972 Lipp 340/1725 PrimaryExaminer-Vincent P. Canney Attorney, Agent, or FirmE. H. Duffield [57]ABSTRACT A reel-to-reel direct tape drive mechanism is disclosed inwhich the data recorded on the tape is made insensitive to tape velocityvariations during read and write operations by converting each codedcharacter to be recorded into a unique bit count. which is recorded sothat only a counting operation is necessary to read data recorded on thetape.

8 Claims, 3 Drawing Figures 21 EWTTER l READ/WR/TE EQ WEMU B15018CONTROL READ A i READ WRITE- CONTROL 1/ REGISTER READ TAPE R01 CONTROLCONTROL Paramznwszm 3.831.196

SIEH 1G 2 TAPE CASSETTE NRZI MAGNETIC PATTERN BIT PATTERN ON TAPE 1 1 10 0 0 0 1 1 1 1 1 1 1 0 0 0 0 PATENTEDA SIEEI 2G 2 FIG. 2 WRITE I/2wRIIEREG'STER) REGISTER 2 r j 7 1/2 WRITE 0 FROM A a: REG. EMPTY/ RMEaSSATGEROR A E E a WRITE i 1 VHEX EMITTER READ/WRITE FROM EMITTER DISC 18CONTROL READ A 7 HEAD Tfi L II READ WRITE CONTROL '1/2 READ READREGISIER REGISTER) I4 16 B To A A- MESSAGE 8 -BUFFER m: 15 II 4 1 /20 MsERRoR 5- CONTROL 4 1 EOT BACKSPACE TAPE WRITE MOTOR READ LCONTROL vREWIND CONTROL 19 SEARCH MAGNETIQ TAPE RECORDING WTI-IOD AND APPARATUSFIELD OF THE INVENTION This invention relates to magnetic mediarecording apparatus and techniques in general and reel-to-reel directdrive recording apparatus and to encoding methods and techniques inparticular.

PRIOR ART In the field of data handling and communications, andparticularly in the field of computer applications, efficient uses ormagnetic recording media to store data in the form of encoded bits havebeen widely devel oped. In general, a premium in design and developmenteffort has been placed upon increasing the density of data recorded andincreasing the speed and accuracy with which the data may be recorded onand read from the media. An inherent problem with all of the prior artapproaches is that, as the density of recorded information increases,careful speed control and tape alignment procedures and techniquesbecome mandatory in order to prevent skewing of the tape, misreading ofinformation, or a complete loss of data during a read or write operationdue to strictly mechanical fluctuations in speed and tape movement. Verysophisticated tape drive mechanisms have been developed to maintainprecise tape velocities and alignments and equally sophisticated startand stop mechanisms have been developed to reduce the lag betweenstart-up of tape and beginning of a read operation. With the advent ofmodern data processing techniques and machines in commerical andbusiness usage the need for a portable data recording device has becomeincreasingly apparent in such fields as inventory taking, list keeping,and related remotely collected batch type data handling operations.Similarly, the need for an inexpensive, reliable and easily manufactureddata input/output device suitable for use in a commercial environmenthas also been apparent. One of the major drawbacks with the use of priorart systems is their great cost and mechanical and electrical complexitynecessitated by the sophisticated drive and sense mechanisms utilized.

OBJECTS OF THE INVENTION In view of the foregoing shortcomings andlimitations inherent in the prior art tape recording methods andapparatus, it is an object of this invention to provide an improvedspeed insensitive method of recording and reading data on a magneticmedia.

It is a further object of this invention to provide an improved datarecording apparatus of low cost and high reliability which requiressimple and unsophisticated drive mechanisms.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an overall view of areel-to-reel tape cassette drive mechanism.

FIG. 2 illustrates, in schematic form, the apparatus of a preferredembodiment of the invention utilizing the recording method of theinvention.

FIG. 3 illustrates an example of recording patterns produced by themethod of the invention.

FIG. 1 illustrates a reel-to-reel tape transport apparatus adapted todrive a cassette'holding two reels on which magnetic recording tape isalternately wound and unwound in accordance with the direction ofrotation thereof. No tape drive capstan is utilized to drive the tape,rather, the reels are driven directly by shafts I and 2 which, in turn,are driven by main drive shaft 3 connected to the motor. Such a driveapparatus may be built using either a frictional drive as illustrated,or gear driven planetary elements. Such mechanisms are well-known in theart as illustrated, for example, by US. Pat. No. 3,528,309. A timingdisc I8 is schematically illustrated as mounted on motorshaft 3 and maybe one of several types including a notched wheel for interrupting alight beam to a photosensor, a toothed magnetic wheel for magneticreluctance proximity sensing, or a magnetically encoded wheel formagnetic sensing of marks. Motor 4 (in FIG. 2) is a reversible typewhich may be driven in either direction during the read, rewind, search,backspace, or write operations.

Data in the form of coded characters (for example coded in EBCDIC,ASCII, or BCD) is to have a magnetic representation thereof recorded onand read from the magnetic tape. While a tape cassette apparatus isillustrated, it is obvious that magnetic discs, full size reel-to-reeltapes, or other magnetic media would be equally employable in thecurrent invention.

The general method of recording and reading data according to theinvention is to translate the coded characters for recording on the tapeinto a bit count, which count is then recorded directly as a number ofbits on the media. Turning now to FIG. 2, an example of write and readoperations will be given following a brief introduction to the systemillustrated. Data to be recorded on the magnetic media is assumed toreside in a record buffer capable of transmitting data characters ondemand for recording. The record buffer is not illustrated for the sakeof simplicity, but could consist of a multi-stage shift register havingstorage space for the requisite number of bits to define each characterin the specific type of character code utilized in the data system. InFIG. 2, motor l is illustrated having output shaft 3 which may be drivenin either direction as dictated by the motor control 5. Assuming that awrite command is received by motor control 5, motor 4 would be energizedto turn in a given direction at an appropriate recording speed. Motorcontrol 5 is for a bidirectional, variable speed motor such as 4. Readand write operations normally occur at the same speed with the tapemoving in the same direction. Search mode is a high-speed readoperation. Rewind is a reverse high speed operation. Back-space is anormal speed reverse operation over one record so that the record may bechecked or re-written. The search mode utilizes high speed forwardreading to check for a desired address, or, in this case, a specificsequence of code. Appropriate speed controls and directional mechanisms,such as the mechanism illustrated in FIG. ll, may be used as desired.

A short delay following the giving of a write command and the start ofactual data recording is provided by a delay control associated with thewrite register 6 so that there will be sufficient time for the motor tocome up to speed, during which, of course, tape is moving from reel toreel. The read-write control ll contains the read amplifiers and writecircuitry. When in write status the pulses from emitter control 8energize the write circuit so that the NRZI magnetic pattern is createdon tape. When in read status, the flux changes on the tape are convertedinto suitable signals for use by the digital logic. The read-writecontrol 11 also has a steering or gating function so that the halfcharacters are assembled properly into the read register 115 and alsohas the logic to recognize end-of-count so that the 1/2 Read register 13may be emptied.

Tape control 19 recognizes the end-of-tape and be ginning-of-tape sothat the normal tape indexing function can be controlled. Also, if aread error should occur, the CRC error control, 24 function could causethe tape to backspace over the record for correction purposes. Such tapecontrols are already well-known in the art, as are CR C c y clisredundancy check) checking techniques, and hence these are not discussedfurther since it is obvious to those of skill in the art that anysuitable tape control and data checking techniques may be used withoutprejudice to the present invention.

Emitter control 8 provides the basic timings for the apparatus. It isvery similar to an electronic timing control where the reference timingis furnished by an oscillator. The reference timing for the emittercontrol is furnished by the emitter wheel 18 whose rotational velocityis directly proportional to the tape velocity and the drive motor speed.This system allows wide ranges in tape velocity since the emittercontrol timing is proportional to tape velocity. Except for this, it issimilar in all respects to a purely electronic control and timingfunction which is well-known in the art for controlling similar typefunctions.

Assuming that ordinary binary code format is utilized, a singlealphanumeric character is placed in write register 6 as illustrated inFIG. 2, from the message buffer. For purposes of description, assumethat this code is 1010011. (equivalent to a decimal count of 41). ThisBCD code will then be encoded into a decimal count code in a two-stepoperation as follows: First, the high order bits (101) are transferredthrough OR gate 7 by appropriate gating circuitry controlled by a pulsefrom emitter control 8 which, in turn, is controlled by the emitterwheel i3 attached to shaft 3. The gating of the stages in a particularstorage register is well-known in the art and is not illustrated for thesake of clarity. Then, these high order bits are placed in the one halfwrite register 9 at the next pulse from emitter control 8. One-halfwrite register 9 is a binary counter which is incremented once for eachensuing pulse from emitter control 8 and a single one bit is recorded onthe tape as each count of this register occurs. The recording andcounting continue until one half write register 9 is empty. in the caseillustrated, it takes three count pulses to clear register of itscontent (lltll therefore, three successive one bits will be recorded onthe tape with each consecutive pulse coming from emitter disc 18 throughthe emitter control 8 as follows: the initial write command conditionsone leg of AND gate it). The other two legs of AND gate 110 areconditioned by the non-zero state of register 9 and by the presence ofan emitter pulse from emitter control 8, and the output from AND gate isfed through the read-write control lll which pulses the read-write headT2 to record a onebit on the tape.

The second step of a write operation consists of transferring the loworder bits from register 6 into register 9 and, after an arbitrarynumber of pulses from emitter control 8 have passed, register 9 isincremented as described above, and additional successive one-bits arerecorded as before. H0. 3 shows in chart form the relationships whichexist during the recording of this assumed example. In the top line ofFIG. 3, regularly spaced emitter control pulses are illustrated whichresult from the pickup from the emitter wheel 18. The magnetic patternof flux transitions encoded on the tape are applied in the well-knownnon-return to Zero (NRZl) recording technique and are illustrated belowthe emitter pulse configuration in FIG. 3. The bit pattern on the tapeis illustrated below the magentic pattern in conjunction with thespecific emitter pulses in the assumed example. As illustrated, the lasthalf of the writing operation results in seven one-bit being recorded onthe tape since seven count pulses are required to clear register 9 of a(MDT content of binary code. The intervening zero bits between the firstand second half of the write operation have no significance in the bitpattern illustrated, and serve only as a data separation function for aread operation to be described below. This procedure continues until thecontents for the write register 6 are completely written and the writeregister is empty. A longer sequence of zero bits would be inserted atend of the record for record separation. In the event that a code to bewritten happens to contain lltltl when it is loaded into one-half writeregister 9, eight one-bits will be written. This is done, for example,by emitter control 8 incrementing one-half write register 9 once andthen checking for an empty condition, thereby overcoming the problem ofan initial write load containing all zero-bits. If emitter control 8 isbuilt to check for the empty condition of one-half write register 9 onlyafter incrementing it at least once, and after every incrementthereafter, no difficulty exists.

An example of a read operation will now be given. First, the tape ismoved back to its starting point by reversing motor 4 and driving thetape backwards until a BOT (beginning of tape) indicator is reached andsensed by tape control 19 as is well-known in tape indexing arts. Uponsensing the BOT indicator, motor 4 is stopped and a read command isapplied to motor control 5 which starts motor 4- tuming in the oppositedirection from the rewind. Again, an appropriate delay is included toallow the drive motor 4 to come up to speed. AND gate Zll is alsoconditioned by the read command as illustrated. The first one bitdetected by read head 12 is amplified and shaped by the read-writecontrol ill and entered into the one-half read register 13, which isfirst set to an all ones condition at the start of any read operation,and this first one bit will clear register 13 to an all zero condition.The next one bit read will result in a 0M content of register 13. If thefirst data on the tape was llllltltltltlllllllllltltltltl as describedpreviously, the third one bit would cause register T3 to go to a 0mstatus binary since these pulses are entered into register 113 as abinary count. During the reading of these first three successive onebits, emitter control 8 is interrogated by the read-write control lll tosee how many emitter pulses go by between the one bits which are readfrom the tape. In an ideal system, there would be a one-for-onecorrespondence between one bits and emitter pulses, but in the actualsystem, zero, one, or two emitter pulses may go by between one hits asthey are read. This is caused by eccentricities in die drive mechanism,slipping and tape stretch. Arbitrarily, it is assumed that if at leastthree emitter pulses are detected between tape onebits which are read,then the end of a count has been reached. These numbers for assumptionsare completely arbitrary and may be changed at the option of thedesigner and are only used for illustration purposes; for example, tenzero bits could be recorded for data separation and five zero bits couldbe recognized as the end of a count. In the present case, end of countis recognized after 111 is read from tape because of the detection of atleast three zero bits between one bits. 010 would result in register 13.The contents of register 13 are inverted by inverter 14 and stored inthe high order half of read register 15 as dictated by the read-writecontrol 11 conditioning the appropriate AND gate 16. 010 will appearthen, in read register 15 and 101. Register 13 is again set to all onebits (111) and reading continues. Similarly, seven successive singlebits will be counted into the read register 13, the first bit clearingit to zero, and its status will be 110. This count will, in turn, beinverted and placed in the lower half of read register 15 through ANDgate 17 as 001 in its inverted state. The contents of read register 15will now be 101001, the original binary code assumed in the example. Thecontent of read register 15 can then be placed in the message buffer(not illustrated) one byte at a time for use by the using system. In theexample just described, one-half write and one-half read steps wereutilized to conserve the amount of recording tape used. A single fullwrite or full read step would be entirely compatible with the novelmethod set forth, but, if a typical 64-character code set of datacharacters were used, it would require 64 counts (or one bits) to recordthe worst case of (000000). This would result in 64 onebits or pulsesrecorded on the tape and, on the average, it would require 36.5 bitspaces on the tape to record the average character code count, assumingequal usage distribution of characters in the data stream. This figureis derived figuring 64 one bits for recording 000000 plus four zero bitspaces for separation of the character, one one-bit for the bestpossible case of 111111, and four more zero bits for separation of thatcharacter for a total of 73 bit spaces, divided by two to find theaverage, which is 36.5.

By using a two-step operation with one-half write and one-half readsteps, the worst case 000000 requires only 24 bit spaces (eight one bitswritten twice plus four separating zero bits written twice). The lestnumber of bit spaces in this case would be a single one-bit plus fourseparating zero bits written twice for the code 111111. The averagenumber of bit spaces in this method would be 17, assuming equal usage ofcharacters and an arbitrary four zero bits for separation of characters.Alternative numbers of steps are arbitrarily choosable. For example,three one-third write or onethird read steps could be used, requiring anaverage of 19.5 bit spaces per character, figured using the method 55above. Other similar fractional read and write options are choosable atwill, but, for six-bit binary codes and the assumed character spacing offour zero hits, the optimum exists at one-half read and one-half writesteps operation as illustrated.

It will be easily appreciated that the system and method described arecompletely insensitive to velocity variations in the drive system forthe tape transport since it is only required to sense the presence of aone bit somewhere on the tape in order to count it. Conversely, ofcourse, this method utilizes a good deal of 65 space on the tape anddoes not place a premium on high density recording. This, however, isnot the object of the present invention since adequate storage spaceexists on 200 feet of tape which may be wound in a typical tapecassette, such as that produced commercially by the Phillips Co, toallow for the recording of up to 65,000 binary characters which issufficient for many commercial applications. This assumes a recordlength of 100 characters and the use of a single track head. A doubletrack head system would yield over 100,000 binary characters.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A method of recording multi-bit coded data char acters on magneticmedia, comprising the steps of:

converting each said multi-bit coded data character 0 into a uniquenumber of pulses, said number being representative of said datacharacter; and serially recording magnetic representatiops of saidnumber of pulses on said magnetic media. 2. A magnetic media datastorage and retrieval 5 method for handling multi-bit coded datacharacters, comprising the steps of:

converting each said multi-bit coded data character into a unique numberof signal pulses, said number being representative of said datacharacter; serially recording magnetic representations of said number ofsignal pulses sequentially on a magnetic medium; retrieving data fromsaid magnetic medium by sens ing said recorded signal pulses, countingsaid pulses sensed; and

reconverting said count of said sensed pulses into a character in acoded data format. 3. The method of claim 2, wherein: said convertingand said reconverting steps are each performed separately for both thelower and higher order portions of said coded data characters formattedin multi-bit coding schemes. 4. Apparatus for magnetically recordingdata as a number of pulses, comprising:

receiving and temporary storage means for holding a coded data characterto be recorded;

conversion means for converting said coded data character into a uniquenumber of pulses, said number being representative of said datacharacter; and

recording means for placing said pulses on a magnetic record media.

5. Apparatus as described in claim 4, further comprising:

sensing means for sensing the presence of said recorded pulses on saidmagnetic media;

reconverting means connected to said sensing means for recoverting saidpulses into coded data characters.

6. Apparatus as described in claim 5, wherein:

said conversion means comprises a binary counter for holding said datafor conversion and a source of stepping pulses for stepping said counterto zero;

said recording means comprises a magnetic recording head and controlcircuitry therefore which is responsive to said conversion means torecord one pulse on said magnetic media for each stepping of saidcounter; and

said reconverting means comprises a binary counter connected to saidsensing means for counting the 8 wherein:

said converting and recording steps are performed simultaneously.

8. A magnetic media storage and retrieval method as number of saidpulses sensed and an inverter means described in claim 2, wherein:

for inverting said count when said pulses have been counted.

7. A method of recording multi-bit coded data characters on magneticmedia, as described in claim 1,

said converting and recording steps are performed simultaneously andsaid reconverting and counting steps are performed simultaneously.

1. A method of recording multi-bit coded data characters on magneticmedia, comprising the steps of: converting each said multi-bit codeddata character into a unique number of pulses, said number beingrepresentative of said data character; and serially recording magneticrepresentations of said number of pulses on said magnetic media.
 2. Amagnetic media data storage and retrieval method for handling multi-bitcoded data characters, comprising the steps of: converting each saidmulti-bit coded data character into a unique number of signal pulses,said number being representative of said data character; seriallyrecording magnetic representations of said numbeR of signal pulsessequentially on a magnetic medium; retrieving data from said magneticmedium by sensing said recorded signal pulses, counting said pulsessensed; and reconverting said count of said sensed pulses into acharacter in a coded data format.
 3. The method of claim 2, wherein:said converting and said reconverting steps are each performedseparately for both the lower and higher order portions of said codeddata characters formatted in multi-bit coding schemes.
 4. Apparatus formagnetically recording data as a number of pulses, comprising: receivingand temporary storage means for holding a coded data character to berecorded; conversion means for converting said coded data character intoa unique number of pulses, said number being representative of said datacharacter; and recording means for placing said pulses on a magneticrecord media.
 5. Apparatus as described in claim 4, further comprising:sensing means for sensing the presence of said recorded pulses on saidmagnetic media; reconverting means connected to said sensing means forrecoverting said pulses into coded data characters.
 6. Apparatus asdescribed in claim 5, wherein: said conversion means comprises a binarycounter for holding said data for conversion and a source of steppingpulses for stepping said counter to zero; said recording means comprisesa magnetic recording head and control circuitry therefore which isresponsive to said conversion means to record one pulse on said magneticmedia for each stepping of said counter; and said reconverting meanscomprises a binary counter connected to said sensing means for countingthe number of said pulses sensed and an inverter means for invertingsaid count when said pulses have been counted.
 7. A method of recordingmulti-bit coded data characters on magnetic media, as described in claim1, wherein: said converting and recording steps are performedsimultaneously.
 8. A magnetic media storage and retrieval method asdescribed in claim 2, wherein: said converting and recording steps areperformed simultaneously and said reconverting and counting steps areperformed simultaneously.